Respiratory disease in horses significantly impacts both the well-being of the infected animals, as well as their financial value to their owners. The major etiological agent for respiratory disease in horses is equine influenza virus (EIV), a type A influenza virus of the orthomyxovirus family.
Eight RNA segments contain the entire EIV genome. This genome encodes a matrix protein (MP) that surrounds the RNA segments, a nucleoprotein (NP) that coats the RNA segments, and a complex of polymerase enzymes that together with NP serves to transcribe and replicate the viruses within the nucleus of an infected cell. A lipid bilayer virion envelope encloses the entire viral structure. The EIV genome also encodes two main surface glycoproteins, hemagglutinin (HA) and neuraminidase (NA), that project out from the lipid bilayer. HA serves to attach the virus to the host cell, whereas NA acts on the mucus bonds within the respiratory mucus blanket to permit the virus access to the underlying epithelial cells. NA also aids in the release of replicated virus from the infected cell. In response to EIV infection, the infected equine host cell generates antibodies against HA that can prevent attachment of the virus to the cell. [See in general, Myers and Wilson, Clinical Techniques in Equine Practice, 5 (3):187-196 (2006)].
Due to the risk of sudden and widespread outbreaks of respiratory disease caused by the highly contagious EIV, vaccines containing HA antigens (e.g., killed EIV isolates) have been developed and successfully employed in equine. Of the sixteen HA and nine NA subtypes of influenza viruses, the H7N7 (A/equine/1) and the H3N8 (A/equine/2) are the only combinations that have been identified in horses. Moreover, H3N8 is by far the predominant combination found in diseased horses [Myers and Wilson, Clinical Techniques in Equine Practice, 5 (3):187-196 (2006)]. However, because influenza virus is an RNA virus, the HA protein is particularly susceptible to genetic and antigenic drift, which consequently leads to vaccine failure. Therefore, there is a great need to identify new EIV isolates that that when administered in vaccines to equine provide protection against currently emerging EIV strains, e.g., in the U.S. especially, for use in new safe and efficacious EIV vaccines.
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